The present invention relates to a current-access magnetic bubble device of dual-conductor type which has a propagation path capable of carrying out bubble propagation using a low driving current and is capable of stable retention of bubbles even when the current is off.
Magnetic bubble devices are expected to provide high-performance, high-density, low-cost, and non-volatile memories. Among these devices, the current-access device not only provides better performance enhancement but also is more economical than the field-access device. Further, the current-access device of dual-conductor type enjoys an inherent superior feature over single-conductor current-access devices which are inferior in design flexibility.
However, the current-access magnetic bubble devices dissipate excessive power due to the direct application of a driving current to the conductor pattern, and consequently, generate too much heat to permit a high-bit-density memory.
While magnetic bubble devices are characterized by non-volatility, i.e. the stored information remains in the memory even if the driving current is removed, the conventional propagation circuit cannot reliably retain bubbles in a stable condition when the driving current is off, and as a result, bubbles may sometimes divert from the regular propagation path under disturbance. This may cause distortion or drop out of the recorded information.
Techniques intended to enhance the performance, versatility and functional capabilities of magnetic bubble devices have been described in many reference materials such as: "Current-Access Magnetic Bubble Circuits" on pp. 1453-1540 in the Bell System Technical Journal, Vol. 58, No. 6, July-August 1979, published by American Telephone and Telegraph Company.
There has been proposed another method of reducing the driving current by providing a permalloy keeper on the conductor (the third layer) of a current-access device, but no report of a device embodying the method has been disclosed to date (paper titled "Can Perforated-Sheet Devices Be Scaled Down From 2 to 0.5 Micron Lithography" presented by B. S. Han et al. at the International Conference on Magnetic Bubbles. ICMB-4, Sept. 26, 1980.)